There have been many proposals including a method of producing a porous film with communicated voids generated therein by stretching, in a uniaxial or biaxial direction, a film produced by formulating an inorganic filler such as calcium carbonate in a film-producing resin such as a polyolefin resin. Many proposals have been offered to use these porous films in various fields such as sanitary materials, medical materials, building materials, agriculture sheets and battery separators.
Although calcium carbonate is a filler which is inert and difficult to wear away production equipments, it has a hydrophilic surface and is therefore highly hygroscopic and less affinity to resins. Therefore, these drawbacks are improved by surface treatment. However, there is a fear that an insufficient performance is only obtained for the reason that calcium carbonate has inferior dispersibility in resins when it is used in sophisticated applications.
Various studies have been made to improve this problem as to inferior dispersibility. For example, a trial has been made to dehumidify a calcium carbonate filler by adding quicklime. However, quicklime has a problem concerning micronization difficulty and also, has the problem that it tends to abrade a kneader and a molding apparatus more easily than calcium carbonate. Also, though studies as to the addition of a dehydrating agent such as vinyl silane are being made, it poses a problem as to a cost-up factor.
Meanwhile, examples of reasons of inferior dispersion include various reasons caused by a filler, for example, deficiency or excess of affinity between a resin and a filler and uneven surface treatment by a filler. However, when water already exists on the surface of a calcium carbonate particle, the particles are easily coagulated and therefore, surface treatment using a hydrophobic organic material cannot be sufficiently performed.
This is the cause why water and low-molecular organic substances (low-boiling point) in a resin composition are vaporized by heating during kneading with a resin, leading to the formation of silver streaks and gas marks, which hinders film formation or the cause of an immixture of abnormally large voids in voids formed on the porous film to be obtained even if a film is formed successfully.
Also, polyester resins such as bioplastics, PET and PEN tend to be hydrolyzed by water. Generally, defects such as gas marks are regarded as problems if water content in a resin composition (compound) is more than 100 ppm.
These problems may be solved if calcium carbonate which has lower water content, or is less hygroscopic, or is easily dried can be provided. For instance, it is generally known that calcium carbonate is surface-treated after wet grinding, resulting in reduction of water content. However, this process essentially needs dehydration and drying steps and therefore brings about disadvantages on cost. Also, when specific surface area is increased, water content is increased, which limits the use thereof. Also, there is, for example, a method in which quicklime is added as a dehydrating agent to adsorb water. This method, however, raises the pH of the resin composition and is therefore limited in its use. Also, its reactivity makes it difficult to micronize it and its coarse particles pose a problem when it is used, particularly, in film fields.
It has been reported so far that when a light calcium carbonate filler is produced by heating to a powder temperature of 200 to 350° C. using high-speed stirring at a peripheral speed of 20 m/s or more to dehydrate to a level of a water content of 0.02% by weight or less, followed by adding a fatty acid metal soap or nonionic surfactant to mix the both at a temperature more than the melting point of the soap or surfactant and less than 180° C., and the obtained filler is formulated in a plastic and injection-molded to obtain a molded article 4 mm in thickness, the molded article is resistant to generation of a silver mark caused by water and is greatly improved in impact resistance, surface glossiness and heat resistance (Patent Document 1).